How disappointed would Withers be if I cast Speak with Dead on him?

Not quite.

Spoiler Warning!!

!It's heavily implied by the game, although not outright stated, that Withers is in fact Jergel himself. He is THE Scribe of the Dead and the one who ascended Bane, Bhaal, and Myrkul to godhood in the first place. Thus is ultimately responsible for all the calamity befalling the Sword Coast and it's the reason he is helping you to begin with. !<